This invention relates to an uninterruptible power system, a parallel operation-type uninterruptible power system, an inverter unit and a parallel operation-type inverter unit.
Of conventional uninterruptible power systems, a so-called continuous inverter-type uninterruptible power system is constructed so as to convert AC power provided by an AC power supply into DC power to output it to a load when the commercial power supply is in a normal condition and so as to convert DC power provided by a storage battery into AC power to output it to the load when power failure occurs in the commercial power supply. The uninterruptible power system includes an inverter circuit for power conversion constituted by a semiconductor switching element, as well as an inverter circuit control device including a PWM control signal generating means for generating a PWM control signal which subjects the semiconductor switching element to PWM control to permit AC power synchronized with commercial power to be outputted from the inverter circuit while adjusting an output power thereof. When an increase in capacity is required, a plurality of such uninterruptible power systems are connected in parallel to each other for operation. In addition to the uninterruptible power systems, a plurality of inverters for driving such as motors or the like may be subjected to parallel operation. In parallel operation of the plural inverters, an extreme increase in load sharing ratio of each of the inverters may possibly causes breakage of the inverter. Also, it causes actuation of a safety circuit, leading to a failure in parallel operation of the inverters.
Parallel operation of the plural uninterruptible power systems causes a drift current to occur in a reactive current when the uninterruptible power systems are different in output voltage from each other. Also, the parallel operation causes a drift current to occur in an active current as well, when output currents of the power units are different in phase from each other. Thus, the prior art is constructed so as to permit the uninterruptible power system to equitably share power by extracting both a reactive current and an active current from an output current of each of the uninterruptible power systems to vary an amplitude of an output voltage thereof depending on a variation in reactive current and vary a phase of the output voltage depending on a variation in active current. Alternatively, in the prior art, equitable feeding of power to the plural uninterruptible power systems is carried out by detecting a sum of currents of the plural uninterruptible power systems fed to the load to compare the sum with a current of each of the uninterruptible power systems, resulting in carrying out control so as to permit the current to be an average of the plural uninterruptible power systems.
Japanese Patent Publication No. 40704/1994 discloses techniques of subjecting a plurality of inverters connected in parallel to each other to parallel operation, wherein an output of a differential amplifying circuit for amplifying a differential voltage between an output voltage of the inverter and a sinusoidal reference wave is used for controlling the inverters. A means for adding or subtracting an output current of the inverter to or from an input of the differential amplifying circuit or an output thereof is provided. Thus, when it is desired to reduce an output voltage of the inverter, the output current of the inverter is added to the sinusoidal reference wave from a sinusoidal wave generating circuit and then fed to the differential amplifying circuit together with the output voltage of the inverter. When it is desired to increase the output voltage, the output current and the output voltage are added thereto and are fed to the differential amplifying circuit together with the sinusoidal reference wave, resulting in controlling a drift current between the inverters. The parallel operation thus constructed permits each of the inverters to solely carry out controlling of a drift current thereof without considering outputs of the remaining inverters.
However, the prior art wherein parallel operation of the plural uninterruptible power systems connected in parallel to each other requires operation of each of the power failure-free power units depending on outputs of the remaining uninterruptible power systems, so that it is required to prepare or arrange any additional circuit for associating the uninterruptible power systems with each other. Thus, mere parallel connection of the plural uninterruptible power systems to each other fails in satisfactory operation of the uninterruptible power systems. The above-described approach disclosed in Japanese Patent Publication No. 40704/1994 fails to prevent an extreme increase in load sharing of each of the inverters although it controls the drift current.
The present invention has been made in view of the foregoing disadvantages of the prior art.
Accordingly, it is an object of the present invention to provide a parallel operation-type uninterruptible power system which is capable of preventing an extreme increase in load sharing of each of uninterruptible power systems constituting the parallel operation-type uninterruptible power system without requiring to operate each of the uninterruptible power systems while considering operation of the remaining uninterruptible power systems.
It is another object of the present invention to provide an uninterruptible power system which is capable of being used for the above-described parallel operation-type uninterruptible power system.
It is a further object of the present invention to provide a parallel operation-type inverter unit which is capable of preventing an extreme increase in load sharing of each of inverter units without requiring to operate each of the inverter units while considering operation of the remaining inverter units.
It is still another object of the present invention to provide an inverter unit which is capable of being used for the above-described parallel operation-type inverter unit.
In accordance with one aspect of the present invention, an uninterruptible power system is provided. The uninterruptible power system includes an inverter circuit for converting DC power converted from AC power of a commercial power supply into AC power to output it to a load when the commercial power supply is in a normal condition and converting DC power of a storage battery into AC power to output it to the load when the commercial power supply falls into a power failure condition. The inverter circuit is constituted by semiconductor switching elements. The uninterruptible power system also includes an inverter circuit control device including a voltage detecting means for detecting an output voltage of the inverter circuit, a current detecting means for detecting an output current of the inverter circuit, and a PWM control signal generating means for generating a PWM control signal for subjecting the semiconductor switching elements constituting the inverter circuit to PWM control to permit the inverter circuit to output the AC power synchronized with commercial power supply while adjusting an output voltage thereof. The PWM control signal generating means includes a differential amplifying circuit for amplifying a difference between the output voltage and a sinusoidal reference signal and is constructed so as to generate the PWM control signal depending on a signal obtained by adding or subtracting the output current to or from an input or output of the differential amplifying circuit in such a manner that the output current is subtracted from the sinusoidal reference signal.
In the present invention generally constructed as described above, the inverter circuit control device further includes an operation means for operating a supply active power being fed to the load by the inverter circuit depending on the output voltage and output current and a judging means for judging whether the supply active power operated by the operation means reaches a predetermined level before it falls into an overload condition. Also, the PWM control signal generating means generates the PWM control signal so as to gradually reduce the supply active power or gradually reduce the output voltage of the inverter circuit as supply active power increases and until the judging means judges that the supply active power reaches the predetermined level and so as to reduce the output voltage at a reduction ratio increased as compared with before to keep it from falling into the overload condition when the judging means judges that the supply active power reaches the predetermined level. Such construction of the present invention, when the uninterruptible power systems are operated while being connected in parallel to each other, permits an initial load sharing ratio of each of the uninterruptible power systems to be determined depending on a magnitude of an output voltage thereof determined depending on construction thereof. More specifically, the uninterruptible power system increased in output is increased in load sharing ratio. Such a condition causes occurrence of a drift current due to a difference between the output voltages of the uninterruptible power systems. However, the drift current does not adversely affect operation of the uninterruptible power system unless it is extremely or abnormally increased. A problem due to the drift current may be solved by techniques disclosed in Japanese Patent Publication No. 40704/1994 owned by the assignee. In order to permit the disclosed techniques to be applied to the present invention, the PWM control signal generating means includes the differential amplifying circuit for amplifying a difference between the output voltage and a sinusoidal reference signal and is constructed so as to generate the PWM control signal depending on a signal obtained by adding or subtracting the output current to or from an input or output of the differential amplifying circuit in such a manner that the output current is subtracted from the sinusoidal reference signal.
Thus, the present invention permits controlling of the drift current during normal operation of the uninterruptible power system. However, when a load sharing ratio of the uninterruptible power system increased in load sharing ratio is increased to a level of the overload condition, the uninterruptible power system is caused to be broken. In order to avoid such a trouble, the present invention is constructed in such a manner that the PWM signal generating means generates the PWM control signal so as to gradually reduce the output voltage of the inverter circuit as supply active power increases and until the judging means judges that the supply active power reaches a predetermined level and so as to reduce the output voltage at a reduction ratio increased as compared with before to keep it from falling into the overload condition when the judging means judges that the supply active power reaches the predetermined level. Such construction, when one of the uninterruptible power systems is reduced in output voltage, permits another uninterruptible power system most increased in output voltage to be increased in load sharing ratio, so that the above-described control may be carried out in the remaining uninterruptible power systems. When it is not required to substantially reduce the output voltage in the remaining uninterruptible power systems, the parallel operation is continued. Thus, the present invention permits each of the uninterruptible power systems to vary its load sharing ratio by itself without monitoring outputs of the remaining uninterruptible power systems, to thereby prevent any one of the uninterruptible power systems from falling into an overload condition. This results in the uninterruptible power systems being subjected to parallel operation without requiring any circuit for associating the uninterruptible power systems with each other.
In this instance, the PWM control signal generating means may be constructed so as to subtract a value obtained by multiplying the supply active power by a first gain constant from an effective value command value for an output voltage to be obtained before the judging means judges that the supply active power reaches the predetermined level and to subtract a value obtained by multiplying the power factor by a second gain constant larger than the first gain constant from the effective value command value for the output voltage to be obtained when the judging means judges that the supply active power reaches the predetermined level.
Also, in accordance with another aspect of the present invention, an inverter circuit is provided. The inverter unit includes an inverter circuit for converting AC power of a commercial power supply into DC power to output it to a load. The inverter circuit is constituted by semiconductor switching elements. The inverter unit also includes an inverter circuit control device including a voltage detecting means for detecting an output voltage of the inverter circuit, a current detecting means for detecting an output current of the inverter circuit, and a PWM control signal generating means for generating a PWM control signal for subjecting the semiconductor switching elements constituting the inverter circuit to PWM control to permit the inverter circuit to output the AC power synchronized with commercial power while adjusting an output voltage thereof. The PWM control signal generating means includes a differential amplifying circuit for amplifying a difference between the output voltage and a sinusoidal reference signal with each other and is constructed so as to generate the PWM control signal depending on a signal obtained by adding or subtracting the output current to or from an input or output of the differential amplifying circuit in such a manner that the output current is subtracted from the sinusoidal reference signal.
In the inverter unit of the present invention generally constructed as described above, the inverter circuit control device further includes an operation means for operating a supply active power being fed to the load by the inverter circuit depending on the output voltage and output current and a judging means for judging whether the active power operated by the operation means reaches a predetermined level before it falls into an overload condition. Also, the PWM control signal generating means generates the PWM control signal so as to gradually reduce the supply active power or gradually reduce the output voltage of the inverter circuit with an increase in the supply active power until the judging means judges that the supply active power reaches the predetermined level and so as to reduce the output voltage at a reduction ratio increased as compared with before to keep it from falling into the overload condition when the judging means judges that the supply active power has reached the predetermined level.
A plurality of the inverter units may be connected in parallel to each other to constitute a parallel operation-type inverter unit. The parallel operation-type inverter unit carries out parallel operation without requiring any circuit for associating the inverter units with each other for such reasons as described above.